Module fabrication of solar cells with low resistivity electrodes

ABSTRACT

One embodiment of the present invention provides a solar module. The solar module includes a front-side cover, a back-side cover, and a plurality of solar cells situated between the front- and back-side covers. A respective solar cell includes a multi-layer semiconductor structure, a front-side electrode situated above the multi-layer semiconductor structure, and a back-side electrode situated below the multi-layer semiconductor structure. Each of the front-side and the back-side electrodes comprises a metal grid. A respective metal grid comprises a plurality of finger lines and a single busbar coupled to the finger lines. The single busbar is configured to collect current from the finger lines.

RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.14/153,608, Attorney Docket Number P67-1NUS, entitled “MODULEFABRICATION OF SOLAR CELLS WITH LOW RESISTIVITY ELECTRODES,” byinventors Jiunn Benjamin Heng, Jianming Fu, Zheng Xu, and Bobby Yang,filed 13 Jan. 2014, which claims the benefit of U.S. ProvisionalApplication No. 61/751,733, Attorney Docket Number SSP13-1001PSP,entitled “Module Fabrication Using Bifacial Tunneling Junction SolarCells with Copper Electrodes,” by inventors Jiunn Benjamin Heng,Jianming Fu, Zheng Xu, and Bobby Yang, filed 11 Jan. 2013.

BACKGROUND

1. Field

This disclosure is generally related to the fabrication of solar cells.More specifically, this disclosure is related to module fabrication ofbifacial tunneling junction solar cells.

2. Related Art

The negative environmental impact of fossil fuels and their rising costhave resulted in a dire need for cleaner, cheaper alternative energysources. Among different forms of alternative energy sources, solarpower has been favored for its cleanness and wide availability.

A solar cell converts light into electricity using the photovoltaiceffect. There are several basic solar cell structures, including asingle p-n junction, p-i-n/n-i-p, and multi-junction. A typical singlep-n junction structure includes a p-type doped layer and an n-type dopedlayer. Solar cells with a single p-n junction can be homojunction solarcells or heterojunction solar cells. If both the p-doped and n-dopedlayers are made of similar materials (materials with equal band gaps),the solar cell is called a homojunction solar cell. In contrast, aheterojunction solar cell includes at least two layers of materials ofdifferent bandgaps. A p-i-n/n-i-p structure includes a p-type dopedlayer, an n-type doped layer, and an intrinsic (undoped) semiconductorlayer (the i-layer) sandwiched between the p-layer and the n-layer. Amulti-junction structure includes multiple single-junction structures ofdifferent bandgaps stacked on top of one another.

In a solar cell, light is absorbed near the p-n junction generatingcarriers. The carriers diffuse into the p-n junction and are separatedby the built-in electric field, thus producing an electrical currentacross the device and external circuitry. An important metric indetermining a solar cell's quality is its energy-conversion efficiency,which is defined as the ratio between power converted (from absorbedlight to electrical energy) and power collected when the solar cell isconnected to an electrical circuit.

FIG. 1 presents a diagram illustrating an exemplary solar cell (priorart). Solar cell 100 includes an n-type doped Si substrate 102, a p⁺silicon emitter layer 104, a front electrode grid 106, and an Al backelectrode 108. Arrows in FIG. 1 indicate incident sunlight. As one cansee from FIG. 1, Al back electrode 108 covers the entire backside ofsolar cell 100, hence preventING light absorption at the backside.Moreover, front electrode grid 106 often includes a metal grid that isopaque to sunlight, and casts a shadow on the front surface of solarcell 100. For a conventional solar cell, the front electrode grid canblock up to 8% of the incident sunlight, thus significantly reducing theconversion efficiency.

SUMMARY

One embodiment of the present invention provides a solar module. Thesolar module includes a front-side cover, a back-side cover, and aplurality of solar cells situated between the front- and back-sidecovers. A respective solar cell includes a multi-layer semiconductorstructure, a front-side electrode situated above the multi-layersemiconductor structure, and a back-side electrode situated below themulti-layer semiconductor structure. Each of the front-side and theback-side electrodes comprises a metal grid. A respective metal gridcomprises a plurality of finger lines and a single busbar coupled to thefinger lines. The single busbar is configured to collect current fromthe finger lines.

In a variation on the embodiment, the single busbar is located at acenter of a respective surface of the solar cell.

In a further variation, two adjacent solar cells are strung together bya stringing ribbon weaved from a front surface of a solar cell and to aback surface of an adjacent solar cell. The stringing ribbon is solderedto single busbars on the front and the back surfaces, and a width of thestringing ribbon is substantially similar to a width of the singlebusbar.

In a variation on the embodiment, single busbars of a front and a backsurface of the solar cell are located at opposite edges.

In a further variation, two adjacent solar cells are coupled together bya metal tab soldered to a first single busbar at an edge of a solar celland a second single busbar at an adjacent edge of the adjacent solarcell. A width of the metal tab is substantially similar to a length ofthe first and the second single busbar.

In a further variation, a length of the metal tab is between 3 and 12mm.

In a variation on the embodiment, the multi-layer semiconductorstructure includes a base layer, a front- or back-side emitter, and aback or front surface field layer.

In a further variation, the multi-layer semiconductor structure furtherincludes a quantum tunneling barrier (QTB) layer situated at both sidesof the base layer.

In a variation on the embodiment, the metal grid comprises at least anelectroplated Cu layer.

In a variation on the embodiment, a width of the single busbar isbetween 0.5 and 3 mm.

In a variation on the embodiment, the solar module further includes aplurality of maximum power point tracking (MPPT) devices. A respectiveMPPT device is coupled to an individual solar cell, thereby facilitatingcell-level MPPT.

In a variation on the embodiment, the front-side and the back-sidecovers are transparent to facilitate bifacial configuration of the solarmodule

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 presents a diagram illustrating an exemplary solar cell (priorart).

FIG. 2 presents a diagram illustrating an exemplary double-sidedtunneling junction solar cell, in accordance with an embodiment of thepresent invention.

FIG. 3A presents a diagram illustrating the electrode grid of aconventional solar cell (prior art).

FIG. 3B presents a diagram illustrating the front or back surface of anexemplary bifacial solar cell with a single center busbar per surface,in accordance with an embodiment of the present invention.

FIG. 3C presents a diagram illustrating a cross-sectional view of thebifacial solar cell with a single center busbar per surface, inaccordance with an embodiment of the present invention.

FIG. 3D presents a diagram illustrating the front surface of anexemplary bifacial solar cell, in accordance with an embodiment of thepresent invention.

FIG. 3E presents a diagram illustrating the back surface of an exemplarybifacial solar cell, in accordance with an embodiment of the presentinvention.

FIG. 3F presents a diagram illustrating a cross-sectional view of thebifacial solar cell with a single edge busbar per surface, in accordancewith an embodiment of the present invention.

FIG. 4 presents a diagram illustrating the percentage of power loss as afunction of the gridline (finger) length for different aspect ratios.

FIG. 5A presents a diagram illustrating a typical solar panel thatincludes a plurality of conventional double-busbar solar cells (priorart).

FIG. 5B presents a diagram illustrating an exemplary solar panel thatincludes a plurality of solar cells with a single busbar at the center,in accordance with an embodiment of the present invention.

FIG. 5C presents a diagram illustrating the serial connection betweentwo adjacent solar cells with a single edge busbar per surface, inaccordance with an embodiment of the present invention.

FIG. 5D presents a diagram illustrating an exemplary solar panel thatincludes a plurality of solar cells with a single busbar at the edge, inaccordance with an embodiment of the present invention.

FIG. 6A presents a diagram illustrating the percentages of theribbon-resistance-based power loss for the double busbar (DBB) and thesingle busbar (SBB) configurations for different types of cells,different ribbon thicknesses, and different panel configurations.

FIG. 6B presents a diagram comparing the power loss difference betweenthe stringing ribbons and the single tab for different ribbon/tabthicknesses.

FIG. 7A presents a diagram illustrating one exemplary placement ofmaximum power point tracking (MPPT) integrated circuit (IC) chips in asolar panel with double-busbar solar cells, in accordance with anembodiment of the present invention.

FIG. 7B presents a diagram illustrating one exemplary placement ofmaximum power point tracking (MPPT) integrated circuit (IC) chips in asolar panel with single-center-busbar solar cells, in accordance with anembodiment of the present invention.

FIG. 7C presents a diagram illustrating one exemplary placement ofmaximum power point tracking (MPPT) integrated circuit (IC) chips in asolar panel with single-edge-busbar solar cells, in accordance with anembodiment of the present invention.

FIG. 7D presents a diagram illustrating the cross-sectional view of anexemplary solar module implementing cell-level MPPT, in accordance withan embodiment of the present invention.

FIG. 8 presents a flow chart illustrating the process of fabricating asolar cell module, in accordance with an embodiment of the presentinvention.

In the figures, like reference numerals refer to the same figureelements.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the embodiments, and is provided in the contextof a particular application and its requirements. Various modificationsto the disclosed embodiments will be readily apparent to those skilledin the art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present disclosure. Thus, the present invention is notlimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the principles and features disclosed herein.

Overview

Embodiments of the present invention provide a high-efficiency solarmodule. The solar module includes a bifacial tunneling junction solarcell with electroplated Cu gridlines serving as front- and back-sideelectrodes. To reduce shading and cost, a single Cu busbar or tab isused to collect current from the Cu fingers. In some embodiments, thesingle busbar or tab is placed in the center of the front and backsidesof the solar cell. To further reduce shading, in some embodiments, thesingle Cu busbar or tab is placed on the opposite edges of the front andbackside of a solar cell. Both the fingers and the busbars can befabricated using a technology for producing shade-free electrodes. Inaddition, the fingers and busbars can include high-aspect ratio Cugridlines to ensure low resistivity. When multiple solar cells arestringed or tabbed together to form a solar panel, conventionalstringing/tabbing processes are modified based on the locations of thebusbars. Compared with conventional solar modules based on monofacial,double-busbar solar cells, embodiments of the present invention providesolar modules with up to an 18% gain in power. Moreover, 30% of thepower that may be lost due to a partially shaded solar panel can berecouped by applying maximum power point tracking (MPPT) technology atthe cell level. In some embodiments, each solar cell within a solarpanel is coupled to an MPPT integrated circuit (IC) chip.

Bifacial Tunneling Junction Solar Cells

FIG. 2 presents a diagram illustrating an exemplary double-sidedtunneling junction solar cell, in accordance with an embodiment of thepresent invention. Double-sided tunneling junction solar cell 200includes a substrate 202, quantum tunneling barrier (QTB) layers 204 and206 covering both surfaces of substrate 202 and passivating thesurface-defect states, a front-side doped a-Si layer forming a frontemitter 208, a back-side doped a-Si layer forming a BSF layer 210, afront transparent conducting oxide (TCO) layer 212, a back TCO layer214, a front metal grid 216, and a back metal grid 218. Note that it isalso possible to have the emitter layer at the backside and a frontsurface field (FSF) layer at the front side of the solar cell. Details,including fabrication methods, about double-sided tunneling junctionsolar cell 200 can be found in U.S. patent application Ser. No.12/945,792 (Attorney Docket No. SSP10-1002US), entitled “Solar Cell withOxide Tunneling Junctions,” by inventors Jiunn Benjamin Heng, ChentaoYu, Zheng Xu, and Jianming Fu, filed 12 Nov. 2010, the disclosure ofwhich is incorporated by reference in its entirety herein.

As one can see from FIG. 2, the symmetric structure of double-sidedtunneling junction solar cell 200 ensures that double-sided tunnelingjunction solar cell 200 can be bifacial given that the backside isexposed to light. In solar cells, the metallic contacts, such as frontand back metal grids 216 and 218, are necessary to collect the currentgenerated by the solar cell. In general, a metal grid includes two typesof metal lines, including busbars and fingers. More specifically,busbars are wider metal strips that are connected directly to externalleads (such as metal tabs), while fingers are finer areas ofmetalization which collect current for delivery to the busbars. The keydesign trade-off in the metal grid design is the balance between theincreased resistive losses associated with a widely spaced grid and theincreased reflection and shading effect caused by a high fraction ofmetal coverage of the surface. In conventional solar cells, to preventpower loss due to series resistance of the fingers, at least two busbarsare placed on the surface of the solar cell to collect current from thefingers, as shown in FIG. 3A. For standardized five-inch (five inch byfive inch) solar cells, typically there are two busbars at each surface.For larger, six-inch (six inch by six inch) solar cells, three or morebusbars may be needed depending on the resistivity of the electrodematerials. Note that in FIG. 3A a surface (which can be the front orback surface) of solar cell 300 includes a plurality of parallel fingerlines, such as finger lines 302 and 304; and two busbars 306 and 308placed perpendicular to the finger lines. Note that the busbars areplaced in such a way as to ensure that the distance (and hence theresistance) from any point on a finger to a busbar is small enough tominimize power loss. However, these two busbars and the metal ribbonsthat are later soldered onto these busbars for inter-cell connectionscan create a significant amount of shading, which degrades the solarcell performance.

In some embodiments of the present invention, the front and back metalgrids, such as the finger lines, can include electroplated Cu lines,which have reduced resistance compared with conventional Ag grids. Forexample, using an electroplating or electroless plating technique, onecan obtain Cu grid lines with a resistivity of equal to or less than5×10⁻⁶ Ω·cm. Details about an electroplated Cu grid can be found in U.S.patent application Ser. No. 12/835,670 (Attorney Docket No.SSP10-1001US), entitled “Solar Cell with Metal Grid Fabricated byElectroplating,” by inventors Jianming Fu, Zheng Xu, Chentao Yu, andJiunn Benjamin Heng, filed 13 Jul. 2010; and U.S. patent applicationSer. No. 13/220,532 (Attorney Docket No. SSP10-1010US), entitled “SolarCell with Electroplated Metal Grid,” by inventors Jianming Fu, JiunnBenjamin Heng, Zheng Xu, and Chentao Yu, filed 29 Aug. 2011, thedisclosures of which are incorporated by reference in their entiretyherein.

The reduced resistance of the Cu fingers makes it possible to have ametal grid design that maximizes the overall solar cell efficiency byreducing the number of busbars on the solar cell surface. In someembodiments of the present invention, a single busbar is used to collectfinger current. The power loss caused by the increased distance from thefingers to the busbar can be balanced by the reduced shading.

FIG. 3B presents a diagram illustrating the front or back surface of anexemplary bifacial solar cell with a single center busbar per surface,in accordance with an embodiment of the present invention. In FIG. 3B,the front or back surface of a solar cell 310 includes a single busbar312 and a number of finger lines, such as finger lines 314 and 316. FIG.3C presents a diagram illustrating a cross-sectional view of thebifacial solar cell with a single center busbar per surface, inaccordance with an embodiment of the present invention. Thesemiconductor multilayer structure shown in FIG. 3C can be similar tothe one shown in FIG. 2. Note that the finger lines are not shown inFIG. 3C because the cut plane cuts between two finger lines. In theexample shown in FIG. 3C, busbar 312 runs in and out of the paper, andthe finger lines run from left to right. As discussed previously,because there is only one busbar at each surface, the distances from theedges of the fingers to the busbar are longer. However, the eliminationof one busbar reduces shading, which not only compensates for the powerloss caused by the increased finger-to-busbar distance, but alsoprovides additional power gain. For a standard sized solar cell,replacing two busbars with a single busbar in the center of the cell canproduce a 1.8% power gain.

FIG. 3D presents a diagram illustrating the front surface of anexemplary bifacial solar cell, in accordance with an embodiment of thepresent invention. In FIG. 3D, the front surface of solar cell 320includes a number of horizontal finger lines and a vertical singlebusbar 322, which is placed at the right edge of solar cell 320. Morespecifically, busbar 322 is in contact with the rightmost edge of allthe finger lines, and collects current from all the finger lines. FIG.3E presents a diagram illustrating the back surface of an exemplarybifacial solar cell, in accordance with an embodiment of the presentinvention. In FIG. 3E, the back surface of solar cell 320 includes anumber of horizontal finger lines and a vertical single busbar 324,which is placed at the left edge of solar cell 320. Similar to busbar322, single busbar 324 is in contact with the leftmost edge of all thefinger lines. FIG. 3F presents a diagram illustrating a cross-sectionalview of the bifacial solar cell with a single edge busbar per surface,in accordance with an embodiment of the present invention. Thesemiconductor multilayer structure shown in FIG. 3F can be similar tothe one shown in FIG. 2. Like FIG. 3C, in FIG. 3F, the finger lines (notshown) run from left to right, and the busbars run in and out of thepaper. From FIGS. 3D-3F, one can see that in this embodiment, thebusbars on the front and the back surfaces of the bifacial solar cellare placed at the opposite edges of the cell. This configuration canfurther improve power gain because the busbar-induced shading now occursat locations that were less effective in energy production. In general,the edge-busbar configuration can provide at least a 2.1% power gain.

Note that the single busbar per surface configurations (either thecenter busbar or the edge busbar) not only can provide power gain, butalso can reduce fabrication cost, because less metal will be needed forbusing ribbons. Moreover, in some embodiments of the present invention,the metal grid on the front sun-facing surface can include parallelmetal lines (such as fingers), each having a cross-section with a curvedparameter to ensure that incident sunlights on these metal lines isreflected onto the front surface of the solar cell, thus furtherreducing shading. Such a shade-free front electrode can be achieved byelectroplating Ag- or Sn-coated Cu using a well-controlled,cost-effective patterning scheme.

It is also possible to reduce the power-loss effect caused by theincreased distance from the finger edges to the busbars by increasingthe aspect ratio of the finger lines. FIG. 4 presents a diagramillustrating the percentage of power loss as a function of the gridline(finger) length for different aspect ratios. In the example shown inFIG. 4, the gridlines (or fingers) are assumed to have a width of 60 μm.As one can see from FIG. 4, for gridlines with an aspect ratio of 0.5,the power loss degrades from 3.6% to 7.5% as the gridline lengthincreases from 30 mm to 100 mm. However, with a higher aspect ratio,such as 1.5, the power loss degrades from 3.3% to 4.9% for the sameincrease of gridline length. In other words, using high-aspect ratiogridlines can further improve solar cell/module performance. Suchhigh-aspect ratio gridlines can be achieved using an electroplatingtechnique. Details about the shade-free electrodes with high-aspectratio can be found in U.S. patent application Ser. No. 13/048,804(Attorney Docket No. SSP10-1003US), entitled “Solar Cell with aShade-Free Front Electrode,” by inventors Zheng Xu, Jianming Fu, JiunnBenjamin Heng, and Chentao Yu, filed 15 Mar. 2011, the disclosure ofwhich is incorporated by reference in its entirety herein.

Bifacial Solar Panels

Multiple solar cells with a single busbar (either at the cell center orthe cell edge) per surface can be assembled to form a solar module orpanel via a typical panel fabrication process with minor modifications.Based on the locations of the busbars, different modifications to thestringing/tabbing process are needed. In conventional solar modulefabrications, the double-busbar solar cells are strung together usingtwo stringing ribbons (also called tabbing ribbons) which are solderedonto the busbars. More specifically, the stringing ribbons weave fromthe front surface of one cell to the back surface of the adjacent cellto connect the cells in series. For the single busbar in the cell centerconfiguration, the stringing process is very similar, except that onlyone stringing ribbon is needed to weave from the front surface of onecell to the back surface of the other.

FIG. 5A presents a diagram illustrating a typical solar panel thatincludes a plurality of conventional double-busbar solar cells (priorart). In FIG. 5A, solar panel 500 includes a 6×12 array (with 6 rows and12 cells in a row) of solar cells. Adjacent solar cells in a row areconnected in series to each other via two stringing ribbons, such as astringing ribbon 502 and a stringing ribbon 504. More specifically, thestringing ribbons connect the top electrodes of a solar cell to thebottom electrodes of the next solar cell. At the end of each row, thestringing ribbons join together with stringing ribbons from the next rowby a wider bus ribbon, such as a bus ribbon 506. In the example shown inFIG. 5A, the rows are connected in series with two adjacent rows beingconnected to each other at one end. Alternatively, the rows can connectto each other in a parallel fashion with adjacent rows being connectedto each other at both ends. Note that FIG. 5A illustrates only the topside of the solar panel; the bottom side of the solar panel can be verysimilar due to the bifacial characteristics of the solar cells. Forsimplicity, the fingers, which run perpendicular to the direction of thesolar cell row (and hence the stringing ribbons), are not shown in FIG.5A.

FIG. 5B presents a diagram illustrating an exemplary solar panel thatincludes a plurality of solar cells with a single busbar at the center,in accordance with an embodiment of the present invention. In FIG. 5B,solar panel 510 includes a 6×12 array of solar cells. Adjacent solarcells in a row are connected in series to each other via a singlestringing ribbon, such as a ribbon 512. As in solar panel 500, thesingle stringing ribbons at the ends of adjacent rows are joinedtogether by a wider bus ribbon, such as a bus ribbon 514. Because onlyone stringing ribbon is necessary to connect adjacent cells, comparedwith solar panel 500 in FIG. 5A, the total length of the bus ribbon usedin fabricating solar panel 510 can be significantly reduced. Forsix-inch cells, the length of the single stringing ribbon that connectstwo adjacent cells can be around 31 cm, compared with 62 cm of stringingribbons needed for the double-busbar configuration. Note that such alength reduction can further reduce series resistance and fabricationcost. Similar to FIG. 5A, in FIG. 5B, the rows are connected in series.In practice, the solar cell rows can be connected in parallel as well.Also like FIG. 5A, the finger lines run perpendicular to the directionof the solar cell row (and hence the stringing ribbons) and are notshown in FIG. 5B.

Comparing FIG. 5B with FIG. 5A, one can see that only a minor change isneeded in the stringing/tabbing process to assemble solar cells with asingle center busbar into a solar panel. However, for solar cells with asingle edge busbar per surface, more changes may be needed. FIG. 5Cpresents a diagram illustrating the serial connection between twoadjacent solar cells with a single edge busbar per surface, inaccordance with an embodiment of the present invention. In FIG. 5C,solar cell 520 and solar cell 522 are coupled to each other via a singletab 524. More specifically, one end of single tab 524 is soldered to theedge busbar located on the front surface of solar cell 520, and theother end of single tab 524 is soldered to the edge busbar located onthe back surface of solar cell 522, thus connecting solar cells 520 and522 in series. From FIG. 5C, one can see that the width of single tab524 is substantially the same as the length of the edge busbars, and theends of single tab 524 are soldered to the edge busbars along theirlength. In some embodiments, the width of single tab 524 can be between12 and 16 cm. On the other hand, the length of single tab 524 isdetermined by the packing density or the distance between adjacent solarcells, and can be quite short. In some embodiments, the length of singletab 524 can be between 3 and 12 mm. In further embodiments, the lengthof single tab 524 can be between 3 and 5 mm. This geometricconfiguration (a wider width and a shorter length) ensures that singletab 524 has a very low series resistance. The finger lines, such as afinger line 526, run in a direction along the length of single tab 524.Note that this is different from the conventional two-busbarconfiguration and the single center-busbar configuration where thefingers are perpendicular to the stringing ribbons connecting twoadjacent solar cells. Hence, the conventional, standard stringingprocess needs to be modified by rotating each cell 90 degrees in orderto string two solar cells together as shown in FIG. 5C.

FIG. 5D presents a diagram illustrating an exemplary solar panel thatincludes a plurality of solar cells with a single busbar at the edge, inaccordance with an embodiment of the present invention. In FIG. 5D,solar panel 530 includes a 6×12 array of solar cells. Solar cells in arow are connected in series to each other via a single tab, such as atab 532. At the end of the row, instead of using a wider bus ribbon toconnect stringing ribbons from adjacent cells together (like theexamples shown in FIGS. 5A and 5B), here we simply use a tab that issufficiently wide to extend through edges of both end cells of theadjacent rows. For example, an extra-wide tab 534 extends through edgesof cells 536 and 538. For serial connection, extra-wide tab 534 canconnect the busbar at the top surface of cell 536 with the busbar at thebottom surface of cell 538. For parallel connection, extra-wide tab 534may connect both the top or bottom busbars of cells 536 and 538. Unlikeexamples shown in FIGS. 5A and 5B, in FIG. 5D, the finger lines (notshown) run along the direction of the solar cell rows.

The stringing ribbons or tabs can also introduce power loss due to theirseries resistance. In general, the distributed power loss through seriesresistance of the stringing ribbons increases with the size of the cell.Moreover, using single stringing ribbon instead of two ribbons alsoincreases this series-resistance-induced power loss because thesingle-ribbon configuration means that there is more current on eachribbon, and the power loss is proportional to the square of the current.To reduce such a power loss, one needs to reduce the series resistanceof the stringing ribbon. For the single center-busbar configuration, thewidth of the ribbon is determined by the width of the busbar, which canbe between 0.5 and 3 mm. Hence, one way to reduce the resistance of theribbon is to increase its thickness as thicker ribbons have lowerresistivity. FIG. 6A presents a diagram illustrating the percentages ofthe ribbon-resistance-based power loss for the double busbar (DBB) andthe single busbar (SBB) configurations for different types of cells,different ribbon thicknesses, and different panel configurations. In theexample shown in FIG. 6A, the ribbons are assumed to be Cu ribbons.

From FIG. 6A, one can see that for 200 μm thick ribbons, theribbon-resistance-induced power loss for a five-inch cell with a singlebusbar (SBB) (at the center) configuration is 2.34%, compared to the1.3% power loss of the double busbar (DBB) configuration. To limit thepower loss to less than 2% in order to take advantage of the 1.8% powergain obtained from the reduced shading by eliminating one busbar, thethickness of the single stringing ribbon needs to be at least 250 μm.For larger cells, such as a six-inch cell, the situation can be worse.For the single center-busbar configuration, ribbons with a thickness of400 um are needed to ensure less than 3% power loss in the six-inchcell, as indicated by cells 602 and 604. Note that the number of cellsin a panel also affects the amount of power loss.

400 um is the upper boundary for the ribbon thickness because thickerribbons can cause damage to the cells during the soldering process. Morespecifically, thicker ribbons may result in warping of the cells, whichcan be caused by stress and the thermal-coefficient difference betweenthe ribbon material and the semiconductor material. Moreover,reliability concerns also start to surface if the stringing ribbons aretoo thick. Implementation of ultrasoft ribbons can reduce the stress andwarping issues, but a different stringing scheme is required toeffectively reduce the power loss to less than 2% without giving up thegains made by busbar shading reduction and ribbon cost reduction. Insome embodiments, other methods are used to reduce stress and warping,including but not limited to: introducing crimps or springs within thelength of the stringing ribbon, and spot soldering of the thick ribbon.

For the single-edge-busbar configuration, because the tabs are muchwider and shorter than the stringing ribbon, the amount of power lossinduced by the series resistance of the single tab is much smaller. FIG.6B presents a diagram comparing the power loss difference between thestringing ribbons and the single tab for different ribbon/tabthicknesses. From FIG. 6B, one can see that the power loss due to theseries resistance of the single tab is much smaller compared with thatof the single ribbon, as indicated by column 606. For example, the powerloss caused by the 250 um thick single edge tab is merely 0.73% forfive-inch, 96-cell panel layout, and around 1.64% for six-inch, 60-cellpanel layout. Hence, one can see that, even for the six-inch cell in the72-cell panel, an edge tab with a thickness of 250 um is sufficientlythick that it induces less than a 2% power loss, making it possible toachieve an overall power gain considering the reduction in shading.

One more factor that can affect power output of the solar panel is themismatch among cells, which may be caused by a partially shaded solarpanel. To maximize power output, it is possible to incorporate maximumpower point tracking (MPPT) devices into a solar panel to allow apartially shaded or otherwise obscured panel to deliver the maximumpower to the battery charging system coupled to the panel. The MPPTdevice can manage power output of a string of cells or a single cell. Insome embodiments of the present invention, the solar panel implementscell-level MPPT, meaning that each solar cell is coupled to an MPPTdevice, such as an MPPT integrated circuit (IC) chip.

Implementing MPPT at the cell level makes it possible to recoup up to30% of the power that can be lost due to the mismatch inefficiencies.Moreover, it eliminates cell binning requirements and may increaseyield. This can thus significantly enhance the return of investment(ROI) for the array owners by eliminating the inventory management needsof installers to match panels within a string, as well as reducingwarranty reserves because replacement panels no longer need to bematched to the old system. Cell-level MPPT can also increase theavailable surface area for the installation of a solar array,particularly in situations where there may be structural shading of thearray at certain hours of the day or during certain seasons of the year.This is particularly useful to bifacial modules which may experienceshading at both the front- and back-side. The cell-level MPPT alsoallows more flexibility in the system mounting, making it possible touse 1- or 2-axis trackers, and ground mounting on high diffuse lightbackground. Details about the cell-level MPPT can be found in U.S.patent application Ser. No. 13/252,987 (Attorney Docket No.SSP10-1011US), entitled “Solar Panels with Integrated Cell-Level MPPTDevices,” by inventors Christopher James Beitel, Jiunn Benjamin Heng,Jianming Fu, and Zheng Xu, filed 4 Oct. 2011, the disclosure of which isincorporated by reference in its entirety herein.

FIG. 7A presents a diagram illustrating one exemplary placement ofmaximum power point tracking (MPPT) integrated circuit (IC) chips in asolar panel with double-busbar solar cells, in accordance with anembodiment of the present invention. In the example shown in FIG. 7A,the MPPT IC chips, such as an MPPT IC chip 702, are placed betweenadjacent solar cells. More specifically, the MPPT IC chips can be placedbetween the two stringing ribbons. In some embodiments, the MPPT ICchips can make contact with both stringing ribbons and facilitate theserial connection between two adjacent solar cells.

FIG. 7B presents a diagram illustrating one exemplary placement ofmaximum power point tracking (MPPT) integrated circuit (IC) chips in asolar panel with single-center-busbar solar cells, in accordance with anembodiment of the present invention. Like the example shown in FIG. 7A,the MPPT IC chips, such as an MPPT IC chip 704, are placed between twoadjacent solar cells. In some embodiments, the MPPT IC chips arethree-terminal devices with two inputs from one cell and one output tothe adjacent cell. The two inputs can be connected to the top and bottomelectrodes (via corresponding stringing ribbons) of the first solarcell, and the one output can be connected to the top or bottom electrodeof the adjacent solar cell to facilitate the serial connection betweenthe two cells.

In addition to placing the MPPT IC chips in between adjacent solarcells, it is also possible to place the MPPT IC chips at the cornerspacing between solar cells. FIG. 7C presents a diagram illustrating oneexemplary placement of maximum power point tracking (MPPT) integratedcircuit (IC) chips in a solar panel with single-edge-busbar solar cells,in accordance with an embodiment of the present invention. In theexample shown in FIG. 7C, the MPPT IC chips, such as an MPPT IC chip706, are placed at the corner spacing between solar cells. In someembodiments, the MPPT IC chips are in contact with the single tabs tofacilitate the serial connection between the two adjacent chips. Notethat for the single-edge-busbar configuration, wiring outside of thesolar cell may be needed to connect the front and back electrodeslocated on opposite sides of the solar cell with the two inputs of theMPPT chip.

FIG. 7D presents a diagram illustrating the cross-sectional view of anexemplary solar module implementing cell-level MPPT, in accordance withan embodiment of the present invention. In FIG. 7D, each solar cell insolar module 710 includes a top electrode and a bottom electrode, whichcan be the single center busbars shown in FIG. 7B. Each MPPT IC chipincludes a top input terminal, a bottom input terminal, and a bottomoutput terminal. For example, MPPT IC chip 712 includes a top inputterminal 714, a bottom input terminal 716, and an output terminal 718.Top input terminal 714 and bottom input terminal 716 are coupled to topand bottom electrodes of a solar cell. Output terminal 718 is coupled tothe bottom electrode of the adjacent solar cell. In the example shown inFIG. 7D, the solar cells, such as a solar cell 720, can be double-sidedtunneling junction solar cells.

The solar cells and the MPPT IC chips are embedded within an adhesivepolymer layer 722, which can later be cured. Materials that can be usedto form adhesive polymer layer 722 include, but are not limited to:ethylene-vinyl acetate (EVA), acrylic, polycarbonate, polyolefin, andthermal plastic. Solar module 710 further includes a front-side cover724 and a back-side cover 726. For bifacial modules, both front-sidecover 724 and back-side cover 726 can be made of glass. When adhesivepolymer layer 722 is cured, front- and back-side covers 724 and 726 arelaminated, sealing the solar cells and the MPPT IC chips within, thuspreventing damage caused by exposure to environmental factors. Afterlamination, solar module 710 can be trimmed and placed in a frame 728,and is then ready to be connected to an appropriate junction box.

FIG. 8 presents a flow chart illustrating the process of fabricating asolar cell module, in accordance with an embodiment of the presentinvention. During fabrication, solar cells comprising multi-layersemiconductor structures are obtained (operation 802). In someembodiments, the multi-layer semiconductor structure can include adouble-sided tunneling junction solar cell. The solar cells can have astandard size, such as five inch by five inch or six inch by six inch.In some embodiments, the smallest dimension of the solar cells is atleast five inches. Front- and back-side metal grids are then depositedto complete the bifacial solar cell fabrication (operation 804). In someembodiments, depositing the front- and back-side metal grids may includeelectroplating of Ag- or Sn-coated Cu grid. In further embodiments, oneor more seed metal layers, such as a seed Cu or Ni layer, can bedeposited onto the multi-layer structures using a physical vapordeposition (PVD) technique to improve adhesion of the electroplated Culayer. Different types of metal grids can be formed, including, but notlimited to: a metal grid with a single busbar at the center, and a metalgrid with a single busbar at the cell edge. Note that for theedge-busbar configuration, the busbars at the front and back surface ofthe solar cells are placed at opposite edges.

Subsequently, the solar cells are strung together to form solar cellstrings (operation 806). Note that, depending on the busbarconfiguration, the conventional stringing process may need to bemodified. For the edge-busbar configuration, each solar cell needs to berotated 90 degrees, and a single tab that is as wide as the cell edgeand is between 3 and 12 mm in length can be used to connect two adjacentsolar cells. In some embodiments, the length of the single tab can bebetween 3 and 5 mm.

A plurality of solar cell strings can then be laid out into an array andthe front-side cover can be applied to the solar cell array (operation808). For solar modules implementing cell-level MPPT, the MPPT IC chipsare placed at appropriate locations, including, but not limited to:corner spacing between solar cells, and locations between adjacent solarcells (operation 810). The different rows of solar cells are thenconnected to each other via a modified tabbing process (operation 812),and then electrical connections between the MPPT IC chips andcorresponding solar cell electrodes are formed to achieve a completelyinterconnected solar module (operation 814). More specifically, the topelectrode of a solar cell is connected to one terminal of the IC and thebottom electrode is tied to another terminal of the IC via typicalsemiconducting methods, including, but not limited to: solder bumps,flip chip, wrap through contacts, etc. Subsequently, the back-side coveris applied (operation 816), and the entire solar module assembly can gothrough the normal lamination process, which would seal the cells andMPPT ICs in place (operation 818), followed by framing and trimming(operation 820), and the attachment of a junction box (operation 822).

The foregoing descriptions of various embodiments have been presentedonly for purposes of illustration and description. They are not intendedto be exhaustive or to limit the present invention to the formsdisclosed. Accordingly, many modifications and variations will beapparent to practitioners skilled in the art. Additionally, the abovedisclosure is not intended to limit the present invention.

What is claimed is:
 1. A solar cell, comprising: a multi-layersemiconductor structure; a first electrode positioned on a first side ofthe multi-layer semiconductor structure; and a second electrodepositioned on a second side of the multi-layer semiconductor structure;wherein the first side of the multi-layer semiconductor structure has nomore than one edge substantially covered by a first busbar that is partof the first electrode; and wherein the second side of the multi-layersemiconductor structure has an opposite edge substantially covered by asecond busbar that is part of the second electrode.
 2. The solar cell ofclaim 1, wherein the first electrode comprises a number of finger lines.3. The solar cell of claim 1, wherein the second electrode iscontinuous.
 4. The solar cell of claim 1, wherein a respective electrodecomprises an electroplated layer.
 5. The solar cell of claim 4, whereinthe electroplated layer comprises Cu.
 6. The solar cell of claim 4,wherein the electroplated layer is coated with Ag or Sn.
 7. The solarcell of claim 1, wherein a respective electrode comprises a seed layerformed with a physical vapor deposition technique.
 8. The solar cell ofclaim 7, wherein the seed layer comprises Cu or Ni.
 9. The solar cell ofclaim 1, wherein a width of a respective busbar is between 0.5 and 3 mm.10. A solar module, comprising: a first cover; a second cover; and aplurality of solar cells positioned between the first and second covers,wherein a respective solar cell comprises: a multi-layer semiconductorstructure; a first electrode positioned on a first side of themulti-layer semiconductor structure; and a second electrode positionedon a second side of the multi-layer semiconductor structure; wherein thefirst side of the multi-layer semiconductor structure has no more thanone edge substantially covered by a first busbar that is part of thefirst electrode; and wherein the second side of the multi-layersemiconductor structure has an opposite edge substantially covered by asecond busbar that is part of the second electrode.
 11. The solar moduleof claim 10, wherein two adjacent solar cells are coupled together byelectrically connecting the busbar on a back surface of one of the twoadjacent solar cells with the busbar on a front surface of the other oneof the two adjacent solar cells.
 12. The solar module of claim 10,wherein two adjacent solar cells are coupled together by electricallyconnecting a busbar at an edge of a solar cell and a busbar at anadjacent edge of an adjacent solar cell.
 13. The solar module of claim12, wherein a length of the electrical connection is substantiallysimilar to a length of the first busbar.
 14. The solar module of claim10, wherein the first electrode comprises a number of finger lines. 15.The solar module of claim 10, wherein the second electrode iscontinuous.
 16. The solar module of claim 10, wherein a respectiveelectrode comprises an electroplated layer.
 17. The solar module ofclaim 16, wherein the electroplated layer comprises Cu.
 18. The solarmodule of claim 16, wherein the electroplated layer is coated with Ag orSn.
 19. The solar module of claim 10, wherein a respective electrodecomprises a seed layer formed with a physical vapor depositiontechnique.
 20. The solar module of claim 19, wherein the seed layercomprises Cu or Ni.